Why my combobox doesnt make calcul in the appropriate event

Hi!

I wrote this code and I need to make this operation when I select the first value. But its doesnt make nothing. It works only when I put it this code on a button. I am asking now the question to know if its in a wrong event???

switch (cbxPercentagemFacturacao.SelectedIndex)

{

case 0:

decimal d1, d2, total;

decimal.TryParse(cbxPercentagemFacturacao.Text, out d1);

decimal.TryParse(txtTotal2.Text, out d2);

total = d1 * d2 / 100;

txtValor3.Text = total.ToString("N");

txtTotal3.Text = "";

break;

case 1:

MessageBox.Show("test1");

break;

default:

MessageBox.Show("test2");

break;

}